docs: def fetch_credentials(auth_id, auth_token):
      """Fetches the right credentials either from params or from environment"""
  
      if not (auth_id and auth_token):
          try:
              auth_id = os.environ['PLIVO_AUTH_ID']
              auth_token = os.environ['PLIVO_AUTH_TOKEN']
          except KeyError:
              raise AuthenticationError('The Plivo Python SDK '
                                        'could not find your auth credentials.')
  
      if not (is_valid_mainaccount(auth_id) or is_valid_subaccount(auth_id)):
          raise AuthenticationError('Invalid auth_id supplied: %s' % auth_id)
  
      return AuthenticationCredentials(auth_id=auth_id, auth_token=auth_token)